A vehicle generally comprises a hydraulic brake system for reducing the speed of the vehicle and/or for stopping the vehicle. It needs great effort for manipulating a brake pedal by the driver, so in many vehicles, a brake booster is added in addition to the hydraulic brake device of the hydraulic brake system.
Traditional brake boosters mainly comprise vacuum brake boosters, in which the vacuum in an inlet pipe of an engine is used as a source of booster brake force which generates a vacuum booster force, in the level of several times higher than the pedal brake force, to be applied to a primary brake cylinder. In this way, the primary brake cylinder receives both the pedal brake force and the vacuum booster force, so that the output pressure of the primary brake cylinder is increased, and thus the pedal brake force needed for vehicle braking can be decreased.
The operation of the engine is affected by the operation of the vacuum brake booster since the booster brake force is generated by the vacuum brake booster using the vacuum in the inlet pipe of the engine. Further, after the engine is shut off, no inlet vacuum exists, and thus no booster brake force can be generated.
As a substitution of the vacuum brake booster, an electric brake booster is developed, which drives a booster force transmitting element by an electric motor to generate a booster brake force, independent of the vacuum in the inlet pipe of the engine.
In an electric brake booster, the action of the electric motor shall be coordinated with the action of the brake pedal. For this purpose, sensors are used for sensing the motions of the electric motor and the brake pedal. In general, the electric motor has its own rotor-rotation sensor for monitoring the movement of the electric motor. In addition, a stroke sensor is used for monitoring the displacement of the brake pedal or the displacement of a brake element driven by the brake pedal.
In such an electric brake booster, it is not easy to provide a measurement with high precision by the stroke sensor. Meanwhile, the stroke sensor has low robustness and is easy to be affected by environment factors, like magnetic field, contamination and icing. Further, the stroke sensor is relative expensive.